For many years it has been to conventional practice to fabricate better quality binder covers by using a three-ply construction. Three rigid or semi-rigid rectangular insert panels in spaced relation are sandwiched between two superposed sheets of thermoplastic sheet material and the peripheral edges of the plastic sheets are heat-sealed together. Of the three insert panels generally used in this process, two approximate in size, the cover panels of the binder and the third panel is a narrower insert or strip disposed between the two larger panels to form the back panel or spine of the binder. The outer plastic sheets, which are slightly larger than the inserts, are fused transversely in the areas between the adjacent edges of the back panel insert and the two cover panels as well as about their marginal edges. The transverse seals form the hinges for the binder cover and the peripheral edge seals provide the finished edges thereof. This method is disclosed, for example, in U.S. Pat. No. 3,195,924 dated July 20, 1965 to Carter and U.S. Pat. No. 4,216,046 dated Aug. 5, 1980 to Hackert. Although binder covers of this type have proven satisfactory over the many years of their use, it has been found that the plastic seals on the lower edges of the binders tend to deteriorate in a substantially shorter period of time than the remainder of the outer casing material. The reason for this is that the lower sealed edges of the binder are subjected to significantly greater wear on book shelves, desks and the like. Moreover, the plastic edges, which have been sealed, tend to be more brittle and less durable than the very same plastic material which has not been subjected to such processing.
Another conventional method of fabricating binder covers has been known as the "case made technique", sometimes called "the turned edge construction." As early example of this type of construction was disclosed in U.S. Pat. No. 2,024,881 dated Dec. 17, 1935 to Schade. Three patents to Peterson, et al also disclose this type of casing for books; these are U.S. Pat. Nos. 3,190,678, dated June 22, 1965; 3,215,450, dated Nov. 2, 1965 and 3,277,505, dated Oct. 11, 1966. In this method, an outer covering material is applied onto the outer surfaces of the binder inserts and turned over the outer edges of the inserts where the material is sealed or glued. An inside liner sheet is then applied and glued over the inturned edges of the outer cover sheet. This construction does not, however, lend itself to automated production techniques.
In less expensive loose-leaf binders, covers are sometimes made as a single ply construction. In this type of product, a semi-stiff plastic sheet material is die-cut to form a one piece cover with hinge lines stamped into the plastic sheet. While binder covers of this type have achieved commercial acceptance for some limited purposes, they have not displaced the three-ply construction used in the better quality binders.